I have experienced "back gassing" on a number of occasions, which is associated with excessive wear to the forcing cone or a cylinder/barrel alignment issue or "spitting". The fix for excessive corcing cone wear is for S&W to remove the barrel, turn it down 1 turn and recut the forcing cone. Even lead target loads can cause this problem, if the gun is fired enough. I have seen this happen on both old and new guns.
Back gassing feels like a quick, so fast that it almost didnt happen, "puff", usually on the left side of the face. You can barely feel it with 38 Speicals, but you really know its there with 357s.